Silicone rubber (polyorganosiloxane) is characterized by excellent characteristics including high heat and weather resistance and good mold releasability but it has a serious drawback in that it is inferior in breaking strength, water resistance and gas impermeability as compared to general elastomers, or synthetic rubbers chiefly composed of carbon.
Attempts have, therefore, been made to mix silicone rubber with general elastomers so as to prepare rubber compositions that retain the advantageous features the two components. For example, it has been proposed that the two components be mixed merely by mechanical means as disclosed in JP-A-54-41957 (the term "JP-A" as used herein means an "unexamined published Japanese patent application), JP-A-55-139604 and JP-A-56-76444, or that polyorganosiloxane be bonded to carbon-carbon double bonds (--C.dbd.C--) in elastomers as shown in JP-A-54-157149, JP-B-55-15497 (the term "JP-B" as used herein means an "examined published Japanese patent application), JP-A-55-31817, JP-A-56-76440 and JP-A-56-76441.
In practice, however, it is difficult to obtain uniform mixtures by kneading silicone rubber with general elastomers because of the low compatibility of the two components.
Moreover, components that are not crosslinkable by themselves or those which are not co-crosslinkable with one another are not suitable for blending no matter how good rubber characteristics they exhibit. Even if they can be blended by a certain method, the resulting blends are so poor in their physical properties that the features of the two components are not exhibited to the fullest extent.